Preamble

At the presentation of the siting proposal for the general licence application for a Swiss deep geological repository, Matthias Braun (CEO of Nagra) postulated the following advantages of the “Nördlich Lägern” siting area, which helped it to win the honour of selection over “Jura Ost” and “Zürich Nordost” (see also Nagra 2022 [1]):

“The long-term stability of the geological barrier is greatest in Nördlich Lägern. Protection of the barrier against future erosion is best in this area due to the greatest depth and erosion-resistant rocks overlying the host rock.”

“In Nördlich Lägern, the suitable area and thus the flexibility in the arrangement of the repository is greatest.

In the Nördlich Lägern area there is the largest contiguous area with no evidence of disturbance. This area therefore offers the greatest flexibility for the layout of the camp.”

If one takes a closer look, this argumentation becomes fragile.

3D-seismics

Between autumn 2015 and spring 2017, Nagra carried out a 3D seismic campaign in the Nördlich Lägern siting area. The results of these recordings allow the bedding conditions and tectonic deformation of the rock formations to be mapped in three dimensions. Special features of the rocks can also be identified. The detailed report (Nagra 2019[2]) describes, illustrates and interprets these results in detail.

Figure 1 (Figure 6-13 in NAB 18-35) shows a “Preliminary interpretation of a volumetric dip horizon, Top Opalinus Clay”, which practically corresponds to a morphological map of the surface of this formation. On it are marked (red lines) tectonic faults (fracture structures) and (blue lines) other tectonic lineaments (e.g. folds). In the south (lower part of the mal), the numerous faults of the Baden – Irchel – Herdern fault zone are visible (blue fault zone in Fig. 2). This is followed (in the centre of the map) by a quiet area disturbed only by folds (D1 – D2) and the Strassberg fault (Sb). In the northern part of the mapped area (red fault zone Fig. 2, upper part of the map) the Opalinus Clay is again cut by numerous faults and deformed by small-scale folds. The most important structures are the Siglistorf anticlinal structure (SIA) with its fractures and the Weiach – Glattfelden – Eglisau fault zone (WGE). Due to the strong structuring of this northern area, which appears like a “hill country” on Figure 1, it can be assumed that the Opalinus Clay is strongly fractured between the main faults. However, to make these detail structures visible, a more detailed map scale and probably higher resolution seismic would be necessary.

Figure 1:  «Tentative interpretation of a Top Opalinus Clay volumetric dip horizon slice», which practically corresponds to a morphological map of the surface of this formation (Nördlich Lägern area, Fig. 6-13 from NAB 18-035). Red lines: Tectonic faults, Blue lines: other lineaments. BIH: Baden – Irchel – Herdern fault system; Eg: Eglisau fault; Sb: Strassberg fault; SIA: Siglistorfer anticline; WGE: Weiach – Glattfelden – Eglisau fault. B – B’: Seismic profile Fig. 3.

Figureg 2: The deformation zones and the Bülach threshold  in Fig. 1 (comment: see text).

The vertical displacement of the strata at the Weiach – Glattfelden – Eglisau fault, can locally exceed 30 m (profile A in Fig. 6-11, NAB 18-035). Dislocations of less than 10 m are often not detectable due to the limited resolution of the seismics (Fig. 3), but even in structures interpreted as folds, experience shows that the rock layers in the Jura mountain are not bent, but brittle fractured. This increases water permeability, even in the Opalinus Clay.

At lineament D3, the formation of the sedimentary rocks changes from west to east, from the normal thickness of the marly layers in the Lower Dogger to the so-called “Bülach threshold” (Fig. 2), in which coral limestones were drilled in the deep borehole Bülach 1-1[3].

In the central area of ​​the map (Fig. 2, white areas), the rock formations are hardly disturbed tectonically and only show the usual inclination of a few degrees towards the south.

Figureg 3: 2D seismic profile through the Weiach – Glattfelden – Eglisau fault (WGE) in the cross-section of the Haberstal site (NAB 18-35, Nagra 2019, Fig. 6-11). TLi: Reflector Top Lias, Top: Reflector Top Opalinus Clay). The reflectors in the Opalinus Clay show changes along the entire length of the profile. The main shaft would be located at about the level of point 1520.

The Haberstal site

The disposal facility planned by Nagra consists of surface facilities, of a main shaft and ventilation shafts and the two deep repositories for HLW and ILW (Fig. 4). No inclined nor long horizontal access tunnels are planned from the main shaft to the disposal facility.

As shown in Figures 1 and 2, the Haberstal (Stadel) site is located in the southernmost part of the Weiach – Glattfelden – Eglisau fault zone (WGE). The transition to the Bülach threshold follows immediately to the east. The Opalinus Clay is tectonically deformed at this point, either by the main fault zone or by fractured folds in its foreland.

The Sectoral plan[4] lists various criteria (starting with criterion 1.1 on the spatial extent of the host rock) that must be taken into account in site selection. These criteria exclude disturbed geological conditions that could impair geotechnical properties and increase water pathways. This means that both the construction and operation of a deep geological repository in this zone are hardly conceivable (https://nagra.ch/wp-content/uploads/2022/09/Tiefenlager-beschriftet_Quelle-Nagra.pdf  ). A vertical shaft could be sunk, but it would probably pass through fractured rocks at depth. Transport, service and storage tunnels connecting to the main shaft would also have to cross tectonic faults in the area of influence of the fault zone.

Based on our current state of knowledge and the foreseeably disturbed rock formations at depth, the suitability of the Haberstal site for a deep geological repository, both for HLW and L/ILW, appears at least questionable from today’s perspective.

Figure 4: Disposal facilities, Nagra Haberstal project (https://nagra.ch/wp-content/uploads/2022/09/Tiefenlager-beschriftet_Quelle-Nagra.pdf) . The surface facilities are directly connected to the two-part repository via shafts approximately 800 m deep. The project presented at the presentation of the Haberstal site does not include neither horizontal nor inclined access facilities.

The Haberstal site within the framework of the Sectoral plan

The sectoral plan procedure initiated in 2009 was designed as a site selection procedure for deep geological repositories for the two waste categories of high-level waste (HLW) and low- and intermediate-level waste (ILW).

According to Nagra, this procedure led to the conclusion that the Haberstal site of the Nördlich Lägern was the best site for the preparation of the general licence application. But the situation is obviously not that simple:

• Important questions were left out of the procedure or were dealt with in a purely formal and inadequate manner. These include the question of conflicts of use (geothermal energy and thermal water, hydrocarbons in the Permocarbon Trough; criterion 2.4. according to the sectoral plan).
• Questions and doubts are now also emerging regarding tectonic stress and thus the long-term safety of the site.

The scientific argumentation that led Nagra to this site is not yet available. The individual basic and reference reports are now to be completed and published until the submission of the general licence application. Therefore, the quality of this siting proposal cannot yet be assessed conclusively.

However, the documentation available or publicly accessible to date gives rise to justified doubts as to whether the proposed site can really fulfil the expectations placed in it. We will comment on this and on the numerous institutional questions that this ongoing procedure brings to mind in a further publication.

[1] Nagra 2022: Der Standort für das Tiefenlager. Der Vorschlag der Nagra. Wettingen, 68 S.

[2] Nagra 2019: Preliminary horizon and structure mapping of the Nagra 3D seismics NL-16 (Nördlich Lägern) in time domain. Arbeitsbericht NAB 18-35, Nagra Wettingen, 77 S.

[3] Nagra 2021: Arbeitsbericht TBO Bülach-1-1: Data Report Summary Plot, Arbeitsbericht NAB 20-008, Nagra Wettingen, 19 S.

[4] Swiss Federal Office of Energy SFOE 2008: Sectoral Plan for Deep Geological Repositories, Conceptual Part. file:///Users/wildi/Downloads/8171-Sect_Plan_Geological_Repositories_Print.pdf .